It is of particular interest, especially for reasons of cost and cumbersomeness, to be able to replace large units by a fully electronic circuit of smaller height and volume. A classical solution for supplying direct current to a telephone line consists of using of two symmetrical windings wound on the same magnetic core in opposite winding direction so that, in common mode, only the difference in symmetry of windings causes a differential voltage to appear. The more matched the inductances, the slighter the differential voltage. A symmetrical and fully electronic circuit was described in the above-mentioned parent patent application. Such a circuit, which shall be called "electronic feeding bridge" in the following description, insulates the power source from the point of view of a.c. current from the telephone line, i.e. the feeding bridge presents a high impedance to the telephone line. On the other hand this presents a low impedance to the power source. Thus, if alternating signals exist in the power source, they are transmitted with almost no attenuation to the telephone line through the feeding bridge. Therefore, it is understood how the crosstalk coupling which exists between the various lines of the network by means of the power source is undesirable. It is of particular interest to be able to use a circuit for providing a.c. insulation between the power source and the telephone line to be supplied or in general between a circuit and the source designed to supply the circuit, when both the circuit and the source are the source of alternating signals.